Silver powder is used in the electronics industry for the manufacture of conductor thick film pastes. The thick film pastes are screen printed onto substrates forming conductive circuit patterns. These circuits are then dried and fired to volatilize the liquid organic vehicle and sinter the silver particles.
Many processs currently used to manufacture metal powders can be applied to the production of silver powders. For example, thermal decomposition processes, electrochemical processes, physical processes such as atomization or milling and chemical reduction processs can be used. Thermal decomposition processes tend to produce powders that are spongy, agglomerated, and very porous whereas electrochemical processes produce powders that are crystalline in shape and very large. Physical processes are generally used to make flaked materials or very large spherical particles. Chemical precipitation processes produce silver powders with a range of sizes and shapes.
Silver powders used in electronic applications are generally manufactured using chemical precipitation processes. Silver powder is produced by chemical reduction in which an aqueous solution of a soluble salt of silver is reacted with an appropriate reducing agent under conditions such that silver powder can be precipitated. Inorganic reducing agents including hydrazine, sulfite salts and formate salts can produce powders which are very coarse in size, are irregularly shaped and have a large particle size distribution due to aggregation. Organic reducing agents such as alcohols, sugars or aldehydes are used with alkali hydroxides to reduce silver nitrate. The reduction reaction is very fast; hard to control and produces a powder contaminated with residual alkali ions. Although small in size (<1 μm), these powders tend to have an irregular shape with a wide distribution of particle sizes that do not pack well. It is difficult to control the sintering of these types of silver powders and they do not provide adequate line resolution in thick film conductor circuits.
Therefore, there is a need for a process to produce silver powder spherical particles comprising very small size crystallites and having a d50 paricle size in the range of 0.5 to 3.5 μm to provide silver thick film paste with improved sintering properties.